Method for determining surface coverage by materials exhibiting different fluorescent properties

ABSTRACT

An improved method for detecting, measuring, and distinguishing crop residue, live vegetation, and mineral soil. By measuring fluorescence in multiple bands, live and dead vegetation are distinguished. The surface of the ground is illuminated with ultraviolet radiation, inducing fluorescence in certain molecules. The emitted fluorescent emission induced by the ultraviolet radiation is measured by means of a fluorescence detector, consisting of a photodetector or video camera and filters. The spectral content of the emitted fluorescent emission is characterized at each point sampled, and the proportion of the sampled area covered by residue or vegetation is calculated.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the UnitedStates Government, and may be manufactured and used by or for theGovernment for governmental purposes without the payment of any royaltythereon or therefor.

TECHNICAL FIELD

The present invention relates to luminophor irradiation by anultraviolet source of radiation and more particularly to thediscrimination and quantification of crop residue, live vegetation, andsoil by irradiation thereof with an ultraviolet source.

BACKGROUND ART

Soil erosion from cropland is affected by the areal fraction of the landsurface covered by crop residue, the non-living portion of a crop leftin the field after harvest. Management of crop residues requires leavinga substantial amount of such residues in place in order to minimize soilerosion. As little as 30% coverage of the soil by residue can reducesoil erosion by approximately 90%. Proper crop residue managementconstitutes an important soil and water conservation measure and federallaw requires the Chief of the Soil Conservation Service to maintaintechnical standards and criteria to assure that practices employed meetintended purposes. In order to manage such residues intelligently, arapid, accurate, and objective measurement of the amount of crop residuecover on agricultural land is needed.

Two basic methods are currently employed for measuring crop residue: thephotographic and intercept techniques. They are statistical in nature,require multiple assessments, and are also, to some degree, subjective.

The photographic technique consists of taking single down-lookingphotographs or stereographic pairs of photographs of residue and soiland thereafter manually estimating the fraction of the soil covered byresidue from the photographs. Video cameras and computer-aided analysisof the video images of reflected visible light provide enhancements tothe photographic technique.

The intercept techniques may be grouped into line-transect andpoint-intercept methods. With the line-transect method, measurements ofcrop residue cover are made along the length of a line across the fieldand average residue cover determined statistically. The point-interceptmethod uses a system of cross-hairs, grid points, or dot matrices todefine points where the presence or absence of residue is determined. Atpresent, the line-transect and point-intercept methods are the mostpopular methods used to estimate crop residue cover and many variationsof these methods have been reported. Sometimes line-transect andpoint-intercept methods are combined, e.g., a line is placed and theintercept is read at selected points. Accuracy of this line-pointtransect depends on the length of the line and the number of points usedper line.

The disadvantages of the photographic reflected visible light techniqueare the very slow analysis of the photographs in the laboratory and thespecial equipment required. Furthermore, there is typically a delay ofseveral days from the date of observation until the analysis iscompleted. This delay may not be acceptable for making decisionsregarding residue management.

Although the line-intercept and point-intercept methods and acombination thereof are deemed to be as accurate as the photographicmethod, they are slow, tedious, and somewhat subjective. There is a needfor a method and apparatus which will overcome these disadvantages. Thisinvention will provide the advantages of rapid, accurate, and objectivemeasurement of crop residue cover to an extent unavailable using currentmethods and devices, and will provide the added advantage of a readilytransportable measurement apparatus.

An additional advantage of this invention is its capability to detectlive vegetation, as well as most dead organic matter and to distinguishthese from most soils and, further, to measure the rate of decay of thedead organic material covering the soil. The method of this inventionwill thus be applicable in the field of land assessment, providing ameasure of ground cover by organic material and of stress or damage tothe vegetative cover.

SUMMARY OF THE INVENTION

Accordingly it is an object of this invention to provide an improvedmethod for discriminating among materials which have differentultraviolet fluorescence signatures.

It is also an object of this invention to provide an improved method formeasuring the fraction of the ground at a particular position coveredrespectively by live vegetation, crop residue (dead organic matter), andsoil (composed mostly of inorganic matter).

Another object of the invention is to provide a method for estimatingthe fraction of an entire field covered respectively by live vegetation,crop residue, and soil.

Still another object of the invention is to provide a method that willgive accurate measurements of live vegetation and crop residue cover, ofstress or damage to the vegetative cover, and of the rate of decay ofdead organic material covering soil.

An additional object of the invention is to provide a method that willoffer objective measurements of live vegetation and crop residue cover.

Yet another object of the invention is to provide a method that willoffer rapid measurements of live vegetation and crop residue cover.

A further object of the invention is to provide a method for measuringlive vegetation and crop residue cover that is capable of distinguishingbetween live vegetation, dead vegetation, and mineral soil.

A still further object of the invention is to provide a simpleground-based mobile or hand-held apparatus for measuring live vegetationand crop residue cover.

Briefly, in accordance with this invention, these and other objects areattained by providing a method for analyzing the fraction of a surfacecovered by living plants, dead vegetation, and soil, involving the stepsof irradiating the surface with ultraviolet radiation, detectingfluorescent emission emanating from the surface, characterizing aspectral content of the fluorescent emission, and calculating thefraction of the surface covered by the living plants, the deadvegetation, and the soil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block flow diagram showing the process steps for analyzingthe amount of soil surface covered by living plants and dead vegetation.

FIG. 2 is an cross-sectional view of the apparatus for performing theprocess steps of FIG. 1 of the present invention.

FIG. 3 is a cross-sectional view of the apparatus for performing theprocess steps of FIG. 1 of the present invention showing the imagingcapability of the apparatus.

FIG. 4 is a depiction of the spectrum of fluorescent light emitted bythe illuminated sample.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 generally, the method for analyzing the fraction ofa surface covered respectively by living plants, dead vegetation, andsoil is generally shown in Blocks 1 through 4. Block 1 illustrates thestep of irradiating the surface with ultraviolet radiation. Block 2illustrates the step of detecting fluorescent emission from the surfaceas a result of irradiation. Block 3 illustrates the third step ofcharacterizing the spectral content of the fluorescent emission. Block 4illustrates the fourth step of calculating from the spectral content ofthe fluorescent emission the fraction of the surface coveredrespectively by the live vegetation, the dead vegetation or cropresidue, and the soil.

More specifically, and again referring to FIG. 1, Block 1 illustratesthe first step of illuminating a surface of unknown composition withultraviolet radiation in the 300-400 nm wavelength range. By virtue ofthis illumination with ultraviolet radiation, the surface is excited andcaused to emit fluorescent radiation if it is comprised of certainmolecules. Block 2 illustrates the second step of detecting the quantityof fluorescent radiation emitted by the surface of unknown compositionby utilizing a radiation sensor such as shown in FIG. 2 as numeral 52.Block 3 illustrates the third step of characterizing the spectralcontent of the fluorescent radiation so that, using known spectralcharacteristics of fluorescent emission by live plants, dead vegetation,and soil, the fractional cover of the surface of unknown composition byeach of the respective enumerated constituents can be determined. Block4 illustrates the forth step of calculating the unknown fractional coverby various constituents from the measured spectral characteristics ofthe surface and the spectral characteristics of various live and deadvegetation.

Blocks 5 and 6 illustrate the preferred method of performing the firstand second steps of Blocks 1 and 2. Illuminating the surface of unknowncomposition is preferably modulated and the detecting of the emittedfluorescent radiation from the surface is gated to match the modulationof the illumination by the ultraviolet source. Techniques for suchmodulation are well-known in the art and include flashing theultraviolet source at periodic intervals and accepting fluorescentsignals only for a determined time duration after termination of theflash of the ultraviolet source. Modulating of the source and gating ofthe detection serve to reject stray signals, to separate ambient lightfrom the fluorescent signal and to improve the accuracy to which ameasurement can be made within a determined period of time. Optimizedgating should ultimately allow analysis in the presence of stray lightand possibly from an aircraft.

Block 7 illustrates that the additional step of characterizing thespectral content of the fluorescent radiation may be accomplished simplyby measuring the total amount of fluorescent radiation emitted by thesurface and comparing it with known standards. Alternatively, the amountof fluorescent radiation emitted in each of a plurality of spectralbands can be defined by inserting a filter or a plurality of filters inthe path between the surface of unknown composition and the sensor, asillustrated in Block 7. By selecting the appropriate wavelength bandsand by analyzing the relative spectral contributions of signal in therespective bands, the proportions of the surface of unknown compositioncovered by live and dead vegetation are determined.

The underlying principle of the method is that when plant material isirradiated with ultraviolet radiation, certain molecules absorb the UVphotons and then emit photons of a longer wavelength. Thelonger-wavelength emission is termed fluorescence and occurs in livingplant materials in relatively broad bands with peaks at approximately440, 525, 685, and 740 nanometers (nm). These bands are in the blue,green, red, and near infrared portions of the spectrum, respectively. Byway of example, chlorophyll is responsible for the bands at 685 and 740nm. Riboflavin is responsible for the band at 525 nm. The bands at 685and 740 nm progressively disappear as the plant material dies, so thatdead vegetation fluoresces primarily from 410 to 550 nm. The compoundsthat are responsible for intense fluorescence in vegetation in theviolet-blue-green region are not found in mineral soil. Thereforefluorescence from 410 to 550 nm can be used to distinguish vegetationfrom soil. By measuring fluorescence at 685 and 740 nm, one maydistinguish between live vegetation and crop residue. The theoreticalunderpinnings of the method have been reported by the inventors in theirpapers, "Fluorescence and Reflectance of Crop Residue and Soil," J. Soil& Water Conservation, 48(3), 207-213 (1993), and "Discriminating CropResidues from Soil by Reflectance and Fluorescence Techniques,"IGARSS'93 Digest, 3: 1325-28 (New York: Institute of Electrical andElectronics Engineering, 1993). Both the aforesaid method and theapparatus for carrying out the method as described below have beendescribed in the inventors' article entitled "A Fluorescence Techniquefor the Assessment of Crop Residue," published in Research andTechnology: 1992 R & T Report, Greenbelt, MD: NASA Goddard Space FlightCenter, (Apr. 20, 1993).

In order to assess fractional surface cover by live vegetation or groundresidue, the fluorescent light 34 (in FIG. 2) is quantified. One methodwhich has been demonstrated in the laboratory and has been reported inthe inventors' paper, "Fluorescence and Reflectance of Crop Residue andSoil," cited above, entails comparing the fluorescent flux over theentire 415-550 nm wavelength band between bare soil and soil coveredwith live vegetation or crop residue. Various soils have beencharacterized by the inventors in the laboratory. The 415-550 nmintegrated flux of even the most decomposed crop residue tested was atleast double the intensity of the highest integrated flux for the soilstested.

Measuring fluorescent need not be limited to a single band, as in theexample described in the preceding paragraph. As testing progresses, alook-up table has been developed on the basis of laboratory results, andincreasingly accurate characterization of the fraction of surfacecovered respectively by live vegetation and crop residue will becomepossible as the invention is tested and employed in the field. Soilshave been tested to form a data base for quantification

FIG. 2 generally shows the preferred apparatus for performing the abovedescribed method of FIG. 1. Generally, the preferred apparatus, denotedgenerally by numeral 10, is used for distinguishing various materials 22which exhibit different fluorescent properties and includes a source 30of ultraviolet radiation 32 which induces fluorescent emission 34 in thevarious materials 22. A detecting sensor assembly 38 is coupled tosource 30 of ultraviolet radiation 32. A data acquisition system 43records the quantity of fluorescent emission 34 from the variousmaterials 22 in at least one wavelength band. In this manner, the amountand type of soil cover can be determined by comparison with knownspectral characteristics of live and dead vegetation.

Crop residue meter 10 is powered by power source 12 in the form of a 12or 24 volt battery pack (not shown) or other power sources known in theart. Power source 12 is connected to crop residue meter 10 by anelectrical cable 14. In the preferred embodiment, crop residue meter 10stands about 1.3 meters high and is approximately 30 cm wide, withhandles 16 that provide convenient portability. For operation, cropresidue meter 10 is placed over the sample surface 20. Crop residuemeter 10 is preferably enclosed within a light tight hood 18. Lighttight hood 18 encloses excitation source 30 and sensor assembly 38, andpositions both excitation source 30 and sensor assembly 38 at a fixeddistance, preferably approximately 1 meter, above sample surface 20.Excitation source 30 is preferably an ultraviolet lamp such as a xenonlamp that is approximately 1 inch in diameter and which emits light inthe 300-400 nm wavelength range. Although excitation source 30 ispreferably an ultraviolet excitation lamp, other excitation sources 30may be used, and it may be a monochromatic source of radiation, such asa laser emitting radiation at a wavelength shortward of 370 nm.Excitation source 30 is encased in a source assembly 42 that is about 3cm in diameter and which serves to prevent stray radiation from reachingsensor assembly 38, and is covered by a filter holder 44 which containsa 300-390 nm cut off filter 46. Cut off filter 46 prevents injectioninto sensor assembly 38 of light from excitation source 30 atwavelengths longer than 370 nm which is not due to fluorescence.

Modulation of excitation source 30 is accomplished, in the preferredembodiment, by means of an electromechanical shutter 48 to furtherensure that any light sensed by sensor assembly 38 is due to, and thuscontemporaneous with, illumination by excitation source 30 and not dueto ambient light 50 which leaks in through light tight hood 18. Othermeans of modulating excitation source 30 would include the use of anelectro-optical shutter (not shown) or electrical modulation of theemission of source 30 itself such as by pulsing its electrical power.

Detection of fluorescent light 34 emitted along path 36 toward sensorassembly 38 is accomplished by means of sensor 52 enclosed within sensorassembly 38. Sensor 52 is preferably a photomultiplier tube, but can beany of several means of detecting ultraviolet radiation which arewell-known in the art. Sensor assembly 38 is encased in a tube 39 thatis approximately 3 centimeters in diameter. Sensor assembly 38 furthercontains a filter assembly 40 which contains a plurality of filters 41which can be inserted sequentially between sample surface 20 and sensor52 in order to limit fluorescent light 34 which reaches sensor 52 to aspecific band of wavelengths. Electronic signals from sensor assembly 38are processed by data acquisition system 43, which digitizes, records,and analyzes measurements of fluorescent light 34.

Referring now to FIG. 3, which depicts an alternative embodiment of thisinvention designated generally by numeral 60, alternative embodiment 60similarly contains source assembly 42 which provides for ultravioletirradiation of sample surface 20. Light-tight hood 18 additionallyencompasses sensory assembly 62 which, in this embodiment, contains avideo camera 64 comprised of multiple pixels which are sensitive to thefluorescent light 34 emitted by sample surface 20. Alternativeembodiment 60 operates in the same manner as crop residue meter 10depicted in FIG. 1, with the additional capacity of characterizingmultiple areal elements of sample surface 20.

In operation, the preferred crop residue meter, designated generally bynumeral 10 (in FIG. 2) is placed over a sample surface 20, which may bethe ground when the crop residue meter 10 is being used to measure thefraction of ground surface covered by crop residue 22. Excitation source30 emits a beam of exciting ultraviolet radiation 32 which is used toilluminate sample surface 20. Fluorescent light 34 is emitted by samplesurface 20 in all directions. The spectrum of wavelengths generallyshown as numeral 70 in FIG. 4, which comprises fluorescent light 34, ischaracteristic of the composition of sample surface 20. Excitingultraviolet excitation illumination 32 is characterized by a wavelength71 shortward of 390 nm. A portion of fluorescent light 34 is emittedalong path 36 toward sensor assembly 38. Wavelength components 72, 74,76 of fluorescent light 34 are detected separately by sensor assembly38. In the preferred embodiment illustrated in FIG. 2, this isaccomplished by means of serially inserting a plurality of spectralfilters 40 into path 36 between sample surface 20 and sensor assembly38. The relative quantities of fluorescent light 34 registered at sensorassembly 38 through each of plurality of spectral filters 40 iselectronically logged by data acquisition system 43. Data acquisitionsystem 43 applies a computational algorithm, known to the art ofspectroscopy, in order to derive the fluorescent spectrum of wavelengths70 (shown in FIG. 4) of sample surface 20 and to infer the fraction ofsample surface 20 covered by crop residue 22. Now referring back to FIG.2, the electrical signal from sensor assembly 38 is digitized,processed, recorded, and analyzed electronically by data acquisitionsystem 43.

The operation of crop residue meter 10 is more particularly described asfollows: first, crop residue meter 10 is zeroed. This process involvesmaking sure the readouts are at zero when the system is in the dark. Areading is also taken on a plot of bare mineral soil, this provides abaseline reading for the area to be surveyed. Next the user transportscrop residue meter 10 along a preselected path (not shown) and places iton surface 20 that is to be surveyed at calculated intervals. At eachposition, excitation source 30 is turned on in a modulated manner. Bylimiting the detection to a signal which is synchronous with themodulation of excitation source 30, an increase in signal-to-noise isachieved, and the efficiency of the soil assessment process is therebyenhanced. The temporal window following emission of the excitingradiation 32 to which signal detection is limited may be varied toachieve optimum signal-to-noise. Data acquisition system 43, which maybe an electronic computer, records the relative fluorescent intensity ateach of the locations at which measurements are made, and calculates theproportion of the area sampled (not shown) covered by residue orvegetation 22.

The method taught in the aforesaid description thus provides fordiscriminating among materials which have different ultravioletfluorescent signatures. In particular, live vegetation and crop residuemay be discriminated from bare soil and also from each other. The methodmay be applied in sampling the ground cover of an entire field. Themethod described is rapid in that the device is hand-held and readilyportable, being battery powered and self-contained. The measurement ofground cover provided by the invention is objective in that thealgorithm resident in the electronic computer (not shown) is applied toeach sample surface 20 without subjective evaluation, and is thus anaccurate measurement.

While the specific method and disclosed relates the analysis of soil andvegetation at a particular location, the method is generally applicableto discriminating between differing materials that, when excited by agiven radiation source, fluoresce at differing wavelengths. Obviously,many modifications and variations of the present invention are possiblein light of the above teachings. It is therefore to be understood thatwithin the scope of the appended claims the invention may be practicedotherwise than as specifically described.

We claim:
 1. A method for analyzing the fraction of a surface coveredrespectively by living matter, dead organic matter, and nonorganicmatter, comprising the steps of:(a) irradiating said surface withultraviolet radiation to produce fluorescence; (b) detecting an emittedfluorescent emission emanating from said surface; (c) characterizing aspectral content of said emitted fluorescent emission; and (d)calculating the fraction of said surface covered respectively by saidliving matter, said dead vegetation, and said nonorganic matter.
 2. Themethod as defined by claim 1, wherein said method furtherincludes:modulating said ultraviolet radiation; and detecting saidemitted fluorescent emission emanating from said surface by means of agated detection device.
 3. The method as defined by claim 1, whereinsaid step of (c) includes passing said emitted fluorescent emissionthough at least one spectral filter.
 4. The method as defined by claim1, wherein said step of (c) includes passing said emitted fluorescentemission though a plurality of spectral filters.
 5. The method asdefined by claim 1, in which steps (a) through (c) are applied at aplurality of positions.
 6. The method as defined by claim 5, in which astatistical estimate is performed as to the properties of the ensembleof positions.